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Putting a Spin on Light and Atoms

Photonics.comSep 2010
BERKELEY, Calif., Sept. 17, 2010 — Magnetometers, which measure magnetic fields, come in many shapes and sizes. An ordinary handheld compass is the simplest. Among the most sensitive, however, are alkali-vapor magnetometers, which use light and atoms to sense magnetic fields. Now researchers have made this device even more sensitive by maintaining the spin polarization of atoms for more than 60 seconds at room temperature. This is an improvement of two orders of magnitude over the best previous performance.

In a vapor-cell magnetometer, the spin of a population of atoms is first polarized, as indicated by the vertical red arrow, by a pump laser that is itself circularly polarized. When a magnetic field is applied, the spin vector is rotated, as indicated by the tilted red arrow. (The magnetic field is perpendicular to the plane of this diagram.) The probe laser’s own plane polarization is rotated by the atom’s spin, and the degree of rotation is measured at the detector. (Images: Damon English)
The researchers, who are from the US Department of Energy’s Lawrence Berkeley National Laboratory, the University of California at Berkeley and the Vavilov State Optical Institute in St. Petersburg, Russia, have optimized their alkali-vapor magnetometer by increasing the spin polarization time.

In a spin-polarized population of atoms, more than half the atoms are oriented in the same direction. An alkali-vapor magnetometer polarizes a vapor of alkali-metal atoms — for example potassium, rubidium or cesium — inside a glass cell using a circularly polarized “pump” laser beam.

Because the spinning atoms have a magnetic moment (with north and south magnetic poles, like a bar magnet), an outside magnetic field will tilt the axis of spin and cause it to precess like a spinning top that’s been pushed off the vertical. Changes in the outside field’s strength or direction can be detected using a probe laser to repeatedly measure the vapor’s average spin orientation.
From left, Mikhail Balabas, Todor Karaulanov, Micah Ledbetter and Dmitry Budker with the antirelaxation-coated vapor cell. Inset shows the rubidium reservoir and the lock (red) that can open or close off the interaction area in the bulb.
“The fundamental sensitivity of the measurement depends on a number of variables,” said Dmitry Budker of Berkeley Lab’s nuclear science division, who is also a professor of physics at UC Berkeley. “These include the number of atoms in the sample and, most important, the spin relaxation time of the polarized atoms.”

Spin relaxation is the loss of polarization, the return of the population of atoms to random orientations, which happens faster as atoms collide with other atoms, or if the external magnetic field varies.

“When an alkali-metal atom bounces off a glass wall, it tends to stick for a little while,” said Budker. “During its stay, it is subject to fluctuating magnetic fields, which cause it to lose polarization. So one way to maintain polarization is to keep the atoms away from the wall, or to make their sojourns on the wall shorter.”

One approach is to fill the cell with an inert buffer gas like helium or neon, at a density high enough that the alkali atoms constantly bump into the buffer gas atoms instead of colliding with the walls. The resulting slow diffusion keeps many of the polarized atoms away from the wall for a long time. Nevertheless, collisions with the buffer gas atoms eventually relax the polarization of the metal atoms.

A better way to keep spin coherence high is to coat the interior of the glass vapor cell with an “antirelaxation” coating. The goal is to increase the number of bounces an atom can survive before losing its polarization.

Mikhail Balabas, Todor Karaulanov, Dmitry Budker and Micah Ledbetter in Budker’s laboratory.
“It’s important to reduce magnetic fluctuations by avoiding any heavy atoms in the coating,” he said. Compounds of light carbon and hydrogen atoms are the choice; state-of-the-art antirelaxation coatings are paraffins, known chemically as alkanes. A polarized atom can hit a paraffin coating 10,000 times before losing its polarization.

But Budker and his longtime colleague Mikhail Balabas of St. Petersburg’s Vavilov State Optical Institute have worked to extend relaxation times using different coatings. Contrary to conventional wisdom, Balabas suggested substituting a different kind of hydrocarbon known as an alkene, or olefin. Alkenes are similar to alkanes but, instead of being saturated (all single bonds), have one carbon double bond in the molecule. The researchers’ experiments with rubidium vapor cells subsequently showed that a polarized rubidium atom could bounce off an alkene coating a million times before losing its polarization.

“The coating material is not all there is to prolonging polarization, however,” he said. “One way polarization is lost is when polarized rubidium atoms in the cell get in contact with uncoated surfaces in the cell’s rubidium reservoir – the sidearm that contains a droplet of the solid metal.”
Balabas devised a simple lock – a sliding glass plug that, merely by rotating the cell assembly, opens or closes the stem between the reservoir and the interaction region where the atoms are polarized and measured.

Finally, the researchers slowed spin relaxation due to collisions among the rubidium atoms inside the interaction area of the cell by modifying a technique called SERF (for “spin exchange, relaxation-free”). The physics of SERF were developed by William Happer and applied to magnetometry by Michael Romalis, both of Princeton University. SERF normally uses buffer gas to reduce the number of alkali atoms hitting the cell wall, while at the same time paradoxically stepping up collisions among the alkali atoms themselves, heating the cell to some 150° C and increasing the density of the atomic vapor.

SERF works only for very weak magnetic fields, where precession is slow. Since atoms collide many times during any period of precession, the multiple collisions frequently exchange spin states among the atoms and keep the average polarization high. To extend the relaxation time still further, the Berkeley and Vavilov Institute collaboration used their “super” antirelaxation coating instead of the usual buffer gas.

The experimental setup, built in Budker’s laboratory by Micah Ledbetter and Todor Karaulanov, was designed to maintain fine control over the shape of magnetic fields inside the experimental chamber. The vapor cell was shielded from Earth’s magnetic field by four layers of mu metal, an alloy of nickel and iron that shunts magnetic fields around the shielded area, plus a cylinder of ceramic ferrite.

The experimental assembly was gimballed so the vapor cell could be rotated, letting the sliding plug lock the neck of the flask or unlock it to allow rubidium vapor into the reaction region. Then a circularly polarized pump beam traversed the axis of the experiment to polarize the atomic vapor, while a probe beam passing through the cell from side to side recorded the spin state of the rubidium vapor by measuring how the probe beam’s own linear polarization was rotated.

Three cells were tested, which differed either in construction or in the rubidium isotopes they contained. Relaxation times in two of the cells were about 15 seconds, already a significant extension, but in one, using the most common isotope of rubidium, 85-Rb, the relaxation time stretched to more than a minute. In contrast to the usual SERF setup, this very long relaxation time was achieved at room temperature instead of with extreme heat.

“We have demonstrated two orders of magnitude improvement over the best paraffin coatings, and at room temperature – but at a relatively low magnetic field,” he said. “The next challenge is to use this technique in stronger magnetic fields – as strong as Earth’s magnetic field, for example, where many of the practical applications are.”

At the same time, Budker and his colleagues intend to explore the application of their new coatings, and the other tricks they used to achieve long relaxation times, to devices other than magnetometers. Among the candidates are atomic clocks, quantum memory devices, and other scientific gadgets that likewise depend on long-lived spin polarization of atoms.